This application is based on Japanese Patent Application Nos. 2000-391100 filed Dec. 22, 2000, 2001-241368 and 2001-241369 both filed Aug. 8, 2001, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical waveguide circuit, and more particularly to an optical waveguide circuit which is an optical component used for an optical communication system, and to an optical waveguide circuit with slab waveguides.
2. Description of the Related Art
As the Internet has spread worldwide, it becomes the urgent necessity to construct communication systems that can transmit large amounts of data simultaneously at high speed. As a system that satisfies such a requirement, an optical communication system utilizing optical wavelength division multiplexing (WDM) receives attention, and is being introduced worldwide with the U.S. taking the lead.
To implement optical WDM technique, an optical multiplexer/demultiplexer capable of multiplexing and demultiplexing multiple different wavelengths is absolutely necessary. As one of its actual forms, there is an optical waveguide circuit that implements an optical circuit using optical waveguides on a substrate.
The optical waveguide circuits are ICs in the optical world, which apply the LSI microfabrication technology to form optical waveguides on a planar substrate integrally. Accordingly, they are superior as integrated circuits suitable for mass production, and capable of implementing high performance circuits with complicated circuit configurations. Recently, as the interest in the optical communication system has grown remarkably, the research and development of various materials such as semiconductor, LiNbO3, plastics and silica-based glass has been advanced. Among them, silica-based optical waveguides, which are formed on silicon substrates using silica-based glass, have good matching with silica-based optical fibers constituting transmission lines of optical communications, and can implement stable operation of the optical circuits because of the high stability and long reliability based on the characteristics of the material. In addition, since rectangular cores can be formed at high reproducibility, theory and practice match closely, and high performance circuits with complicated optical circuits can be implemented. With such features, they lead other waveguide materials in practical use.
As basic configurations of the optical multiplexer/demultiplexers using the silica-based optical waveguides with such superior characteristics, there are Mach-Zehnder interferometers and arrayed waveguide gratings. Combining the Mach-Zehnder interferometers with arrayed waveguide gratings can implement various optical multiplexing and demultiplexing characteristics.
The Mach-Zehnder interferometer multiplexes or demultiplexes two different wavelengths, or divides different multiple wavelengths alternately and periodically. FIG. 1A shows a configuration of a Mach-Zehnder interferometer 101. It comprises two optical couplers 102, two optical waveguides 103 interconnecting the two optical couplers 102, and an input waveguide 104 and an output waveguide 105 for connecting it with other optical couplers 102. Here, each optical coupler is composed of a directional coupler consisting of two adjacent optical waveguides. Each optical waveguide is composed of a single-mode waveguide. The spacing between wavelengths to be multiplexed or demultiplexed is set by the waveguide length difference between the two optical waveguides 103.
The arrayed waveguide grating multiplexes and demultiplexes multiple different wavelengths simultaneously. It can achieve such a function with smaller size than achieving it using multiple Mach-Zehnder interferometers. FIG. 1B shows a configuration of an arrayed waveguide grating 111. It comprises two slab waveguides 112, a waveguide array 113 interconnecting the two slab waveguides 112, and an input waveguide 114 and an output waveguide 115 which are connected to different slab waveguides 112. The waveguide array 113, input waveguide 114 and output waveguide 115 are each composed of a single-mode waveguide. The waveguide array 113 comprises adjacent optical waveguides with different waveguide lengths, and the waveguide length difference between the adjacent waveguides determines the spacing between the wavelengths to be optically multiplexed and demultiplexed.
FIG. 2 is a cross-sectional view of a silica-based optical waveguide constituting these optical multiplexer/demultiplexers. It has a structure in which a core 203 is coated with a cladding 202 formed on a substrate 201. The substrate 201 consists of a silicon substrate or a silica substrate, and the cladding 202 and the core 203 are composed of silica-based glass. Using the silicon substrate as the substrate 201 serves as a platform for the hybrid mounting of photo-detectors, light emitting devices and the like. It can also prevent cracks of the cladding 202 and core 203 because of compressive stress imposed on them, thereby increasing reliability.
When using the silicon as the substrate, however, the optical characteristics of the Mach-Zehnder interferometer or the arrayed waveguide grating formed by the silica-based optical waveguides have polarization dependence in which the spectrum in a TM mode with the electric field perpendicular to the substrate shifts to a longer wavelength side as compared with that of a TE mode with the electric field parallel to the substrate.
FIG. 3 and FIGS. 4A and 4B are graphs each illustrating transmission spectra of a Mach-Zehnder interferometer or those of an arrayed waveguide grating. FIG. 3 illustrates transmission spectra of a Mach-Zehnder interferometer with a multiplexing/demultiplexing spacing of 0.8 nm. As illustrated in this figure, the peak wavelengths giving the least loss in the TE mode and TM mode are shifted by about 0.25 nm, and the loss of the other mode is greater about a few dB at each peak wavelength. FIG. 4A illustrates transmission spectra of an arrayed waveguide grating with a multiplexing/demultiplexing spacing of 0.8 nm, and FIG. 4B illustrates enlarged transmission spectra around 1562 nm of FIG. 4A. As in the Mach-Zehnder interferometer, the peak wavelengths that give the least loss for the TE mode and TM mode are shifted by about 0.25 nm, and the loss of the other mode is greater about a few dB at each peak wavelength.
Such polarization dependence in the optical multiplexer/demultiplexer, which makes the polarization direction of a signal light transmitted through an optical fiber indefinite and varied with time, will cause the insertion loss or crosstalk to vary with time, thereby degrading the reliability of the signal.
The polarization dependence of the optical multiplexer/demultiplexer is brought about by the waveguide birefringence of the optical waveguides constituting the circuit, where the effective refractive index of the TM mode perceives is greater than the effective refractive index of the TE mode. The polarization dependence occurs because of the waveguide birefringence of the two optical waveguides 103 in the Mach-Zehnder interferometer, and of the waveguide array 113 in the arrayed waveguide grating.
Defining that the waveguide birefringence of the silica-based optical waveguide is the difference obtained by subtracting the effective refractive index of the TE mode from the effective refractive index of the TM mode, it takes a value of about 2xc3x9710xe2x88x924xe2x88x923xc3x9710xe2x88x924 in the single-mode waveguide. Such a waveguide birefringence results from the compressive stress on the optical waveguide, which is a residual thermal stress caused by the difference between the thermal expansion coefficient of the silicon constituting the substrate and that of the silica-based glass used as the material of the optical waveguide. In addition, since the thermal expansion coefficient varies depending on the type and concentration of the dopant of the silica-based glass, the waveguide birefringence varies even if the same material is used.
Thus, it is essential to eliminate the polarization dependence of the optical multiplexer/demultiplexer to implement it. To eliminate the polarization dependence, the following methods have been developed: (1) a method of reducing the stress imposed on the waveguide by forming grooves on both sides of the waveguide, thereby decreasing the waveguide birefringence; (2) a method of controlling the birefringence by forming a stress-applying film such as a-Si on the waveguide, and by trimming the stress-applying film with monitoring the optical circuit characteristics, thereby reducing the polarization dependence of the entire optical circuit; (3) a method of controlling the birefringence by irradiating ultraviolet rays on the waveguide with monitoring the optical circuit characteristics, thereby reducing the polarization dependence of the optical circuit in its entirety; (4) a method of inserting a half waveplate into the optical circuit to exchange the polarization mode, thereby reducing the polarization dependence of the entire optical circuit; and (5) a method of doping into the cladding covering the core a material that will increase the thermal expansion coefficient such as GeO2, B2O3 and P2O5 to bring it close to the thermal expansion coefficient of the substrate, thereby reducing the birefringence. These methods can reduce the wavelength shift of the foregoing Mach-Zehnder interferometer and arrayed waveguide grating to an order of 0.01 nm. These techniques operate as a birefringence compensator for the optical waveguide circuit.
Although these techniques can implement optical waveguide circuits having polarization dependence eliminated, the following problems still arise to bring them into practical use.
As for the method (1), since the birefringence depends greatly on the position and depth of the grooves, it is sensitive to the fabrication accuracy, thereby reducing the yield and increasing the number of steps of the process. As for the method (2), although the requirement for the fabrication accuracy is not severe, it is unsuitable for the mass production because the optical circuit must be trimmed individually with monitoring the characteristics. As for the method (3), it is also unsuitable for the mass production because the optical circuit must be adjusted individually with monitoring the characteristics. As for the method (4), since the waveguide is cut by an amount equal to or greater than the thickness of the waveplate of more than ten microns to several tens of microns thick, the loss increases. As for the method of (5), when the stress on the cladding becomes the tensile stress, the cladding glass is apt to crack easily, and the weather resistance is decreased because of the heavily doped dopant, resulting in the reduction in the reliability.
Although the polarization dependence is described above which is caused by the waveguide birefringence the Mach-Zehnder interferometer has in its two optical waveguides 103 and the arrayed waveguide grating has in the waveguide array 113, the polarization dependence because of the waveguide birefringence the arrayed waveguide grating has in the slab waveguides cannot be ignored.
Cross-sectional structures of the waveguides of the arrayed waveguide grating are as follows. FIGS. 5A and 5B are views showing a cross-sectional structure of a slab waveguide and a waveguide array: FIG. 5A is a cross-sectional view of a slab waveguide; and FIG. 5B is a cross-sectional view of a waveguide array. A slab waveguide 502 with the cross-sectional view as shown in FIG. 5A has a structure in which a core 503 is coated with a cladding 502 on a substrate 501 with the width of the core 503 being spread in the direction parallel to the substrate. On the other hand, a waveguide array 504 with the cross-sectional view as shown in FIG. 5B comprises single-mode optical waveguides including cores 504 each having a core width nearly equal to the core thickness of the core 503 of the slab waveguide 502. In such silica-based optical waveguides, optical multiplexer/demultiplexer up to 128 waves is implemented.
As described above, as the methods to reduce the polarization dependence of the waveguide array 113, the foregoing methods (1)-(5) have been proposed. The wavelength shift amount of the arrayed waveguide grating, however, varies depending on the output ports of the output waveguide (115 of FIG. 1B). Thus, the conventional techniques for reducing the polarization dependence can little reduce the variations in the wavelength shift amount between the output ports, thereby limiting the reduction in the polarization dependence.
FIG. 6 is a graph plotting the wavelength shift of the TM mode with respect to the TE mode of the output ports of the arrayed waveguide grating that optically multiplexes or demultiplexes 32 light waves with a spacing of 0.8 nm. The light waves are input through input ports 16 of the input waveguide (114 of FIG. 1B). The variations in the wavelength shift amount are about 0.02 nm with a fixed gradient from the output port 1 to 32. The variations are caused by the waveguide birefringence of the slab waveguides 112 of about 1.1xc3x9710xe2x88x923.
Therefore, an object of the present invention is to provide an optical waveguide circuit that can reduce or eliminate its polarization dependence by reducing or eliminating the birefringence with solving the foregoing problems of the yield, mass production, optical characteristics and reliability.
Another object of the present invention is to provide an optical waveguide circuit with reduced polarization dependence caused by the waveguide birefringence of the slab waveguide by reducing the waveguide birefringence.
Still another object of the present invention is to provide an optical waveguide circuit with uniform polarization dependence by reducing the variations in the wavelength shift amount between output ports of the arrayed waveguide grating.
Another object of the present invention is to provide an optical waveguide circuit capable of decreasing the reduction limit of the polarization dependence by combining the present invention techniques with the polarization dependence reduction techniques described in connection with the conventional techniques.
According to one aspect of the present invention, there is provided an optical waveguide circuit including optical waveguides each of which has a cladding and a core formed on a substrate, wherein the optical waveguide circuit comprises at least one of a single-mode waveguide, a multi-mode waveguide and a slab waveguide, wherein at least one of the single-mode waveguide, the multi-mode waveguide and the slab waveguide includes at least in its part a multilayer structure that is composed of multiple types of layers with different refractive indices, and has at least three layers in total, wherein the multilayer structure has first birefringence and second birefringence, the first birefringence deriving from a fact that an effective refractive index of the entire core, which results from contributions of the layers to the refractive index, has different values in directions parallel to and perpendicular to the layers, and the second birefringence deriving from a structure of the waveguide, and wherein when defining a value of the birefringence of the waveguide as a value obtained by subtracting an effective refractive index in the direction parallel to the substrate from an effective refractive index in the direction perpendicular to the substrate, the value of the first birefringence and the value of the second birefringence have opposite signs.
Here, the second birefringence may have a positive value, and the multilayer structure may be composed of layers parallel to the substrate.
The second birefringence may have a negative value, and the multilayer structure may be composed of layers parallel to a light traveling direction and perpendicular to the substrate.
An absolute value of geometrical birefringence caused by the multilayer structure may be less than twice an absolute value of a remaining birefringence value.
Values BSM, BMM and BSL may differ from each other, where BSM, BMM and BSL are geometrical birefringence values caused by the multilayer structure of the single-mode waveguide, that of the multi-mode waveguide and that of the slab waveguide.
The values BSM, BMM and BSL may satisfy a relationship of |BSM| less than |BMM| less than |BSL|.
The refractive indices and thicknesses of the layers constituting the multilayer structure may be substantially symmetrical with respect to a central layer.
The multilayer structure may be composed of two types of layers that have different refractive indices, and are stacked nearly alternately.
As for the layers constituting the multilayer structure, thicknesses of at least layers with highest refractive index may be increased from two end layers adjacent to the cladding to a central layer; thicknesses of at least layers with lowest refractive index may be decreased from the two end layers adjacent to the cladding to the central layer; or the thicknesses of at least the layers with the highest refractive index may be increased from two end layers adjacent to the cladding to a central layer, and the thicknesses of at least the layers with the lowest refractive index may decrease from the two end layers adjacent to the cladding to the central layer. The refractive indices of layers constituting the multilayer structure may increase from two end layers adjacent to the cladding toward inmost part of the core.
The total number of the layers of the multilayer structure may be five to ten.
An average refractive index nave of the core due to the multilayer structure may given by the following equations (2) and (3), and an absolute value of geometrical birefringence Bs caused by the multilayer structure may be give by the following equations (1) and (3):                               "LeftBracketingBar"                      B            s                    "RightBracketingBar"                =                                            c              1                        ⁢                          "LeftBracketingBar"                                                                                          ∑                      i                      N                                        ⁢                                                                  n                        i                        2                                            ⁢                                              q                        i                                                                                            -                                  1                                                                                    ∑                        i                        N                                            ⁢                                                                        q                          i                                                                          n                          i                          2                                                                                                                                "RightBracketingBar"                                +                      c            2                                              (        1        )                                          n          ave                =                              ∑            i            N                    ⁢                                    n              i                        ⁢                          q              i                                                          (        2        )                                          q          i                =                              t            i                                              ∑              i              N                        ⁢                          t              i                                                          (        3        )            
where N is the number of layers of the multilayer structure, ni and ti are refractive indices and thicknesses the individual layers, and c1 and c2 are fixed values determined for each waveguide structure by actual measurement or calculation.
The substrate may consist of one of a silicon substrate, and the waveguide may be composed of silica-based glass.
The substrate may consist of a silica substrate, and the waveguide may be composed of silica-based glass.
The optical waveguide circuit may further comprise an optical interferometer including two optical couplers and a plurality of waveguides with different lengths interconnecting the two optical couplers, wherein as for a waveguide with a minimum waveguide length among the plurality of waveguides, when a length of the multilayer structure of its core is L, and as for the remaining waveguides of the plurality of waveguides, lengths of their multilayer structures equal to L plus differences between lengths of the remaining waveguides and the minimum waveguide length.
The optical waveguide circuit may further comprise a Mach-Zehnder interferometer including two optical couplers and two single-mode waveguides interconnecting the two optical couplers.
The optical waveguide circuit may further comprise an arrayed waveguide grating including: two slab waveguides; a waveguide array consisting of a plurality of single-mode waveguides with different lengths interconnecting the slab waveguides; an input waveguide consisting of single-mode waveguides connected to one of the two slab waveguides; and an output waveguide consisting of single-mode waveguides connected to the other of the two slab waveguides.
The optical waveguide circuit may further comprise a birefringence compensator.
The birefringence compensator may utilize a half waveplate.
Next, a structure common to the single-mode waveguide, multi-mode waveguide and slab waveguide will be described in detail together with a structure associated with the single-mode waveguide and the multi-mode waveguide.
The present invention differs from the conventional techniques in that its core has a multilayer structure composed of multiple layers with different refractive indices. In addition, as for the birefringence of the waveguide, the geometrical birefringence caused by the multilayer structure is canceled out by the other birefringence to reduce or eliminate the birefringence of the waveguide. Specifically, the birefringence of the waveguide can be reduced or eliminated by determining the value of the geometrical birefringence caused by the multilayer structure to have the opposite sign to the value of the other birefringence. Here, placing the magnitude of the geometrical birefringence caused by the multilayer structure at nearly the same value (absolute value) as the magnitude of the other birefringence with the opposite sign makes it possible to bring the birefringence of the waveguide to nearly zero.
The core with the multilayer structure composed of layers with different refractive indices can bring about the geometrical birefringence that will increase the effective refractive index in the direction parallel to the multilayer structure. The magnitude of the geometrical birefringence is determined by the refractive indices and thicknesses of the individual layers. Accordingly, the birefringence of the waveguide can be eliminated by forming the layers in such a manner that they will reduce the waveguide birefringence caused by the residual thermal stress and the like, and by making the magnitude |Bs| of the geometrical birefringence caused by the multilayer structure nearly equal to the magnitude |B0| of the waveguide birefringence caused by the residual thermal stress and the like. Thus, the polarization dependence of the optical waveguide circuit can be reduced. The birefringence can be reduced as compared with the conventional technique by making |Bs| less than 2|B0|. In this case, it is not necessary that the cores of all the optical waveguides of the optical waveguide circuit have a multilayer structure. It is enough that portions of the optical waveguides that will cause the polarization dependence of the optical waveguide circuit are made multiple.
When using the silica-based glass as the material of the optical waveguides and the silicon as the substrate, the optical waveguides undergo the compressive stress. In this case, the effective refractive index increases in the electric field perpendicular to the substrate (TM mode). Therefore, the birefringence of the optical waveguide can be reduced by forming the multilayer structure in the direction parallel to the substrate so that the birefringence caused by the compressive stress is canceled out by the geometrical birefringence due to the multilayer structure. In contrast, the silica substrate will increase the effective refractive index of the optical waveguide in the electric field parallel to the substrate (TE mode). Therefore, the birefringence of the optical waveguide can be reduced by the multilayer structure perpendicular to the substrate.
The single-mode waveguide with a multilayer structure composed of layers with different refractive indices can reduce the birefringence of the waveguide. For example, the polarization dependence of the arrayed waveguide grating is mainly due to the waveguide birefringence of the waveguide array composed of the single-mode waveguides as described above. Accordingly, the optical waveguides of the waveguide array with a multilayer structure can reduce the birefringence of the optical waveguide, thereby being able to reduce the polarization dependence. In addition, the waveguide array composed of optical waveguides, each of which includes a multilayer core with the same order of length as their integral parts, that is, a multilayer core formed in an appropriate part of each optical waveguide having influence on the polarization dependence, can reduce the polarization dependence as compared with the conventional techniques.
In addition, the multi-mode waveguide with the multilayer structure composed of the layers with different refractive indices can reduce the birefringence of the optical waveguide. For example, a multi-mode interferometer (called xe2x80x9cMMIxe2x80x9d from now on) using a multi-mode waveguide with a width several times wider than the single-mode waveguide as its optical coupler has the polarization dependence in the excess loss. However, the polarization dependence in the excess loss can be reduced by the multilayer structure.
In addition, the multilayer structure, which is applied to the cores of both the single-mode waveguide and multi-mode waveguide rather than to the core of only one of them, can further reduce the polarization dependence of the optical waveguide circuit. For example, as for the Mach-Zehnder interferometer using the MMI as the optical couplers, the multilayer structure can be applied to the two single-mode waveguides interconnecting the two MMIs and to the multi-mode waveguides of the two MMIs at the same time to further reduce its polarization dependence.
The waveguide birefringence value can vary depending on the geometry of the optical waveguide such as its dimension. In this case, the multilayer structures with suitable geometrical birefringence values, which are applied to the single-mode waveguide and multi-mode waveguide, respectively, can reduce the polarization dependence of the optical waveguide circuit appropriately. As with the silica-based optical waveguides, the waveguide birefringence is apt to increase with the width of the waveguide. Therefore, the multilayer structure should be formed such that the geometrical birefringence value of the multi-mode waveguide is made greater than that of the single-mode waveguide.
As for the multilayer structure, it is possible to align the center of the distribution of the electromagnetic field of the traveling light close to the center of the core as in the conventional techniques by making the refractive indices and thicknesses of the layers of the multilayer structure core nearly symmetric with respect to the central layer of the layers. In addition, this enables the distribution profile of the electromagnetic field to be nearly symmetric with respect to the core center in both the directions perpendicular and parallel to the substrate. Therefore, the circuit design analogous to that of the conventional techniques can implement desired circuit characteristics.
In addition, stacking at least two layers with different refractive indices alternately makes it possible to consider the refractive indices the light perceives in the direction perpendicular to the layer are nearly constant. As a result, the birefringence of the optical waveguide can be reduced or eliminated with maintaining the distribution of the electromagnetic field at nearly the same profile as that of the conventional one.
In addition, the graded-index, which increases the refractive indices of the individual layers of the multilayer structure from both ends of the cladding to the center of the core, can reduce or eliminate the birefringence of the optical waveguide with implementing a spot size different from that of the conventional optical waveguide.
As for the total number of the layers of the multilayer structure, the multilayer structure of the core with five to ten layers can be implemented easily even by the thick film fabrication process such as flame hydrolysis deposition (FHD). In addition, the connection loss with the optical fibers can also be reduced as will be described later in connection with the embodiments in accordance with the present invention.
The multilayer structure can be determined without any complicated analysis by determining the factors such that they approximately satisfy the following equations (1)-(3).                               "LeftBracketingBar"                      B            s                    "RightBracketingBar"                =                                            c              1                        ⁢                          "LeftBracketingBar"                                                                                          ∑                      i                      N                                        ⁢                                                                  n                        i                        2                                            ⁢                                              q                        i                                                                                            -                                  1                                                                                    ∑                        i                        N                                            ⁢                                                                        q                          i                                                                          n                          i                          2                                                                                                                                "RightBracketingBar"                                +                      c            2                                              (        1        )                                          n          ave                =                              ∑            i            N                    ⁢                                    n              i                        ⁢                          q              i                                                          (        2        )                                          q          i                =                              t            i                                              ∑              i              N                        ⁢                          t              i                                                          (        3        )            
where |Bs| is the geometrical birefringence value due to the multilayer structure, nave is the average refractive index of the core in the multilayer structure, N is the total number of the layers of the multilayer structure, ni and ti are refractive index and thickness of each layer of the multilayer structure, and c1 and c2 are constants (correction coefficients) that are determined by calculation or measurement in accordance with the waveguide structure.
In the right side of equation (1), the term             ∑      i      N        ⁢                  n        i        2            ⁢              q        i            
represents the effective refractive index in the direction parallel to the layers, and the term   1                    ∑        i        N            ⁢                        q          i                          n          i          2                    
represents the effective refractive index in the direction perpendicular to the layers. Accordingly, the difference between them is the geometrical birefringence originating from the multilayer structure. The calculated values of them are associated with the geometrical birefringence value |Bs|, which is obtained by actual measurement or calculation, by the correction coefficients c1 and c2 as in equation (1).
The correction coefficients c1 and c2 mainly depend on the confinement of light due to the waveguide structure (relative refractive index difference and dimension), thereby taking nearly fixed values for the optical waveguides in which the confinement of light is constant. Accordingly, obtaining the correction coefficients by prototyping an appropriate multilayer structure core or by carrying out mode solver of an appropriate multilayer structure core can facilitate the design of the multilayer structure that will satisfy the desired geometrical birefringence.
As for the multilayer structure in which at least two layers with different refractive indices are stacked alternately, the correction coefficient c1=1 nearly corresponds to the case where the core confines the light sufficiently. When the confinement is weaker, the coefficient is mainly set less than one.
Among the optical waveguide circuits, the polarization dependence appears conspicuously in an optical interferometer that interconnects a first optical coupler and a second optical coupler by multiple waveguides with different waveguide lengths. The Mach-Zehnder interferometer and arrayed waveguide grating are a typical example of them. It is possible to provide optical circuits with smaller polarization dependence by reducing the waveguide birefringence of the optical interferometer by means of the multilayer structure core. In the optical interferometer, the polarization dependence of the circuit can be reduced or eliminated by reducing or eliminating the polarization dependence by an amount corresponding to the waveguide length difference. Specifically, the multilayer structure cores are to be formed over the lengths corresponding to the waveguide length differences from the shortest waveguide length in the optical waveguides. Alternatively, the multilayer structure can be constructed over the lengths corresponding to the waveguide length differences plus a fixed length. Specifically, as for the individual waveguides, the portions of the cores to have the multilayer structure can be set as follows. Here, the entire cores of the waveguides can also have the multilayer structure.
In addition, it is not necessary that the multilayer structure core is formed continuously, but that the total waveguide length becomes a prescribed length. The optical interferometer can include an optical interferometer that has its multiple optical couplers such as lattice-form filters connected by the multiple waveguides.
When a plurality of multilayer structures are required, combining the present invention technique with the birefringence compensator of the conventional techniques for eliminating the polarization dependence described above can reduce the number of types of the multilayer structures to be formed. In addition, when it is difficult to fabricate the multilayer structure that will provide predetermined geometrical birefringence, the combination can eliminate the waveguide birefringence by the geometrical birefringence implementable by the multilayer structure. Thus, it is possible to increase the flexibility of selecting structural parameters of the multilayer structure. As an example of reducing the number of types of the multilayer structures to be formed, let us consider an example in which a half waveplate is applied to a Mach-Zehnder interferometer using MMI as its optical couplers. The half waveplate is inserted into the single-mode waveguides between the optical couplers to eliminate the polarization dependence due to the waveguide birefringence of the single-mode waveguides. As for the multilayer structure, only one multilayer structure is formed for the multi-mode waveguide core. In addition, as a method of eliminating the waveguide birefringence by the geometrical birefringence achievable by the multilayer structure that can be fabricated, a material for increasing the thermal expansion coefficient such as GeO2, B2O3 and P2O5 can be doped into the conventional technique cladding, and adjusted to a geometrical birefringence value implementable by the multilayer structure, for example. In this case, the doping must be carried out at such a level that no tensile stress takes place to maintain the reliability at a certain level.
As described above, it is not necessary for the core of the waveguide to have the multilayer structure over its entire length. Thus, a waveguide with a combination of a multilayer structure core and a uniform core can also be used. In such a waveguide, its birefringence can be reduced or eliminated by making the average refractive index of the multilayer structure core equal to the refractive index of the uniform core, and by making the geometrical birefringence value due to the multilayer structure nearly equal and opposite in sign to the birefringence value of the uniform core. Here, it is assumable that the average refractive index of the multilayer structure core is given by equations (2) and (3), and that the geometrical birefringence caused by the multilayer structure value is given by equations (1) and (3).
As described above, the present invention can eliminate or reduce the waveguide birefringence, and hence can eliminate or reduce the polarization dependence of the optical circuit. In addition, the present invention, which forms cores with a multilayer structure, can employ the conventional fabrication method of the waveguides as described above. Accordingly, it can fabricate optical circuits without decreasing its manufacturing yield, mass productivity, optical characteristics and reliability, which are the problems of the foregoing conventional elimination techniques of the polarization dependence. As a result, the present invention can provide a low cost, high performance, high reliability practical optical circuit.
Next, a configuration of the slab waveguide will be described along with a configuration of a combination of the single-mode waveguide and the multi-mode waveguide. The idea of the configuration of the slab waveguide is the same as that applied to the configuration of the single-mode waveguide and the multi-mode waveguide.
According to the present invention, the multilayer structure, which is composed of layers with different refractive indices and is applied to the slab waveguide core, can bring about the geometrical birefringence that has a higher effective refractive index in the direction parallel to the layers. Forming the multilayer structure core in the direction that will compensate for the birefringence (waveguide birefringence) caused by the residual thermal stress and the like can implement a slab waveguide with reduced waveguide birefringence, which is an object of the present invention, thereby reducing the polarization dependence of the optical circuit caused by the waveguide birefringence of the slab waveguide. In particular, the wavelength shift amounts of the individual output ports of the arrayed waveguide grating can be made uniform. In addition, combining the slab waveguide with the conventional polarization dependence reduction techniques (birefringence compensator) can provide an optical waveguide circuit, the reduction limit of the polarization dependence of which is lowered.
In addition, as for the optical waveguide using silica-based glass as its material and silicon as the substrate, the effective refractive index of the optical waveguide is greater in the electric field in the direction perpendicular to the substrate (TM mode). Accordingly, using the multilayer structure in the direction parallel to the substrate enables the waveguide birefringence due to the stress and the geometrical birefringence due to the multilayer structure to be canceled out each other, thereby implementing the slab waveguide with small waveguide birefringence.
In addition, when a silica substrate is used, the effective refractive index of the optical waveguide is greater in the electric field parallel to the substrate (TE mode). Accordingly, a multilayer structure in the direction perpendicular to the substrate can reduce the waveguide birefringence of the slab waveguide. Thus, the polarization dependence of the optical circuit caused by the waveguide birefringence of the slab waveguide can be reduced. In particular, the wavelength shift amounts of the individual output ports of the arrayed waveguide grating can be made uniform.
In addition, in the silica-based optical waveguides, the slab waveguide with the conventional structure has greater waveguide birefringence than the single-mode waveguide with the conventional structure. To form a multilayer structure with the geometrical birefringence that will cancel out the waveguide birefringence, the geometrical birefringence due to the multilayer structure of the slab waveguide core is made greater than the geometrical birefringence due to the multilayer structure of the single-mode waveguide core. Applying the multilayer structures that can eliminate the waveguide birefringence to the slab waveguide and to the single-mode waveguide independently makes it possible to provide a circuit with small polarization dependence. In particular, the arrayed waveguide grating can make uniform the wavelength shift amounts of the individual output ports, and reduce or eliminate the wavelength shift.
In addition, combining the present invention technique with the conventional polarization dependence reduction techniques can provide an optical waveguide circuit that can lower the reduction limit of the polarization dependence.
In addition, the multilayer structure comprising layers with a high refractive index and layers with a low refractive index which are disposed alternately makes it possible to implement a slab waveguide that has the same effective refractive index as the conventional one, and has the refractive index the light perceives in the direction perpendicular to the layers being almost uniform. Thus, optical circuits can be constructed using the slab waveguide with the same geometry as the conventional one.
In addition, as described above, the waveguide birefringence of the slab waveguide is greater than that of the single-mode waveguide in the silica-based optical waveguide. The waveguide birefringence can vary according the geometry such as the dimension of the optical waveguide, and increases with an increase in the core width of the optical waveguide in the silica-based optical waveguide. Accordingly, as for an optical multiplexing/demultiplexing circuit comprising a combination of an arrayed waveguide grating, which is composed of the single-mode waveguide, multi-mode waveguide and slab waveguide, and a Mach-Zehnder interferometer which comprises optical couplers consisting of multi-mode interferometers (called xe2x80x9cMMIxe2x80x9d from now on), using separate multilayer structures, in which the geometrical birefringence due to the multilayer structure increases in the order of the single-mode waveguide, multi-mode waveguide and slab waveguide, can eliminate the waveguide birefringence, and provide a circuit with a small polarization dependence.
In addition, the center of the distribution of the electromagnetic field of the traveling light can be aligned closely to the center of the core as in the conventional devices by making the refractive indices and thicknesses of the individual layers of the multilayer structure of the core nearly symmetry with respect to the central layer of the layers. In addition, this enables the profile of the distribution of the electromagnetic field to be nearly symmetry with respect to the core center in the directions perpendicular and parallel to the substrate. As a result, desired circuit characteristics can be implemented using nearly the same circuit design method as the conventional techniques.
In addition, the graded-index, which increases the refractive indices of the individual layers of the multilayer structure from both ends of the cladding to the center of the core, can reduce or eliminate the birefringence of the optical waveguide with implementing a spot size different from that of the conventional optical waveguide.
As for the total number of the layers of the multilayer structure, five to ten layers can implement the multilayer structure of the core easily even by the thick film fabrication process such as FHD. In addition, the connection loss with the optical fibers can be reduced as will be described in the embodiments in accordance with the present invention.
The multilayer structure can be determined without any complicated analysis by determining the factors such that they approximately satisfy the foregoing equations (1)-(3). As for the multilayer structure in which at least two layers with different refractive indices are stacked alternately, the correction coefficient c1=1 nearly corresponds to the case where the core confines the light sufficiently. When the confinement is weak, it is mainly set less than one.
In addition, the present invention technique combined with the conventional polarization dependence reduction techniques (birefringence compensator) can eliminate the polarization dependence of the optical waveguide circuit. For example, the multilayer structure is applied to the cores of the slab waveguides and multi-mode waveguide in the arrayed waveguide grating and in the Mach-Zehnder interferometer using the MMI as the optical couplers. As for the single-mode waveguide, which brings about the polarization dependence of the optical circuit by the birefringence, the half waveplate is inserted into the waveguide array of the arrayed waveguide grating, and into the optical waveguides between the optical couplers of the Mach-Zehnder interferometer. Thus, the polarization dependence can be reduced.
In addition, it is not necessary for the cores of all the optical waveguides of the optical waveguide circuit to have the multilayer structure. It is enough that portions of the optical waveguides affecting the polarization dependence of the optical waveguide circuit are made multilayer. For example, as for the arrayed waveguide grating, it is enough that the individual waveguides of the waveguide array and the slab waveguides have the multilayer structure. In addition, applying the multilayer structure to part of them can reduce the polarization dependence as compared with the conventional one. For example, providing the individual waveguides of the waveguide array with multilayer cores of the same order, that is, providing multilayer cores to appropriate portions of the optical waveguides affecting the polarization dependence can reduce the polarization dependence.
As described above, according to the present invention, the waveguide birefringence of the slab waveguide can be reduced, and the polarization dependence caused by the waveguide birefringence of the slab waveguide can be reduced. In particular, the wavelength shift of the individual output ports of the arrayed waveguide grating can be made uniform. In addition, combining the present invention technique with the conventional polarization dependence reduction techniques makes it possible to lower the reduction limit of the polarization dependence, thereby providing a high performance optical waveguide circuits. This means that the loss of signals passing through the optical multiplexer/demultiplexer and the fluctuations of the crosstalk thereof are reduced in the optical multiplexer/demultiplexer, and that the reliability is increased of the signals in the optical communication system using the optical WDM technique.
Next, configurations other than the foregoing configurations will be described which are applicable to the single-mode waveguide, multi-mode waveguide and slab waveguide.
In at least one of the multilayer structures formed in the cores of the optical waveguides of the optical waveguide circuit, the graded-index can be implemented which has a refractive index that effectively increases from the end layers to the center of the cladding, and the birefringence of the optical waveguides can be reduced or eliminated together with implementing a different spot size in the following structure: at least layers with higher refractive indices among the individual layers are made thicker from the end layers to the center of the cladding; at least layers with lower refractive indices are made thinner from the end layers to the center of the cladding; and at least layers with higher refractive indices are made thicker from the end layers to the center of the cladding, and at least layers with lower refractive indices are made thinner from the end layers to the center of the cladding.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.